1. Field of the Invention
This invention relates to a motor-driven power steering system for a vehicle adapted to assist the operator-induced steering operation by means of the rotating force of a motor.
2. Description of the Prior Art
In the past, a conventional power steering system of this type, as schematically illustrated in FIG. 6, has hitherto been known in which a steering wheel 1 is operatively connected through a steering shaft 2 and a first rack and pinion gear T.sub.1 including a first pinion 5 and a first rack tooth portion 6a with a steering rack 6 which is connected at its opposite ends with a pair of steerable road wheels (not shown) through a pair of tie rods 8a, 8b so that when the steering wheel 1 is turned by an operator, the steerable road wheels (not shown) are appropriately steered in accordance with the steering motion of the steering wheel 1 induced by the operator. On the other hand, the steering rack 6 is operatively connected with a motor 13 through a second rack and pinion gear T.sub.2 including a second rack tooth portion 6b and a second pinion 18 and a speed-reduction gear R so that the driving force of the motor 13 is transmitted through the speed-reduction gear R and the second rack and pinion gear T.sub.2 to the rack 6 so as to assist the operator-induced steering operation of the steering wheel 1. The motor 13 is electrically connected with a battery 11 through a control unit 9 and a key or ignition switch 12 so that it is energized by the battery 11 under the control of the control unit 9. The control unit 9 is input with control signals from a steering-torque sensor 3 and a vehicle-speed sensor 10 so as to appropriately control the operation of the motor 13 on the basis of the steering torque and the vehicle speed measured.
As shown in FIG. 7, the control unit 9 includes four electrical switching elements 9a through 9d such as, for example, power transistors, power MOS FETs, or the like for controlling the current supplied from the battery 11 to an armature of the motor 13 so as to change the rotational direction of the motor 13, and a control means 9e connected to receive the outputs from the torque sensor 3 and the vehicle-speed sensor 10 for controlling the switching operations of the electrical switching elements 9a through 9d. With this arrangement, the switching elements 9a through 9d have to control the large flow of current supplied to the armature of the motor 13, which is, for example, about 50 to 80 amperes at the time of motor energization, and about 10 to 20 amperes during steady-state operation of the motor 13. Therefore, the conventional power steering system as described above is disadvantageous in that the switching elements must necessarily be costly and large-sized.